internal/atlas: split EndFrame into EndFrame and SwapBuffers

This enables to do something asynchronously while executing SwapBuffers
in a different goroutine.

This is a preparation for HandleInput.

Updates #1704

internal/atlas/image.go

@@ -756,7 +756,7 @@ func (i *Image) DumpScreenshot(graphicsDriver graphicsdriver.Graphics, path stri
 	return i.backend.restorable.Dump(graphicsDriver, path, blackbg, image.Rect(0, 0, i.width, i.height))
 }
 
-func EndFrame(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) error {
+func EndFrame() error {
 	backendsM.Lock()
 	defer backendsM.Unlock()
 	defer func() {
@@ -767,10 +767,6 @@ func EndFrame(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) e
 		panic("atlas: inFrame must be true in EndFrame")
 	}
 
-	if err := restorable.EndFrame(graphicsDriver, swapBuffersForGL); err != nil {
-		return err
-	}
-
 	for _, b := range theBackends {
 		b.sourceInThisFrame = false
 	}
@@ -778,6 +774,22 @@ func EndFrame(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) e
 	return nil
 }
 
+func SwapBuffers(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) error {
+	func() {
+		backendsM.Lock()
+		defer backendsM.Unlock()
+
+		if inFrame {
+			panic("atlas: inFrame must be false in SwapBuffer")
+		}
+	}()
+
+	if err := restorable.SwapBuffers(graphicsDriver, swapBuffersForGL); err != nil {
+		return err
+	}
+	return nil
+}
+
 func floorPowerOf2(x int) int {
 	if x <= 0 {
 		return 0

internal/buffered/image.go

@@ -33,14 +33,6 @@ type Image struct {
 	pixels []byte
 }
 
-func BeginFrame(graphicsDriver graphicsdriver.Graphics) error {
-	return atlas.BeginFrame(graphicsDriver)
-}
-
-func EndFrame(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) error {
-	return atlas.EndFrame(graphicsDriver, swapBuffersForGL)
-}
-
 func NewImage(width, height int, imageType atlas.ImageType) *Image {
 	return &Image{
 		width:  width,

internal/restorable/images.go

@@ -56,7 +56,7 @@ var theImages = &images{
 	shaders: map[*Shader]struct{}{},
 }
 
-func EndFrame(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) error {
+func SwapBuffers(graphicsDriver graphicsdriver.Graphics, swapBuffersForGL func()) error {
 	if debug.IsDebug {
 		debug.Logf("Internal image sizes:\n")
 		imgs := make([]*graphicscommand.Image, 0, len(theImages.images))

internal/ui/context.go

@@ -19,7 +19,6 @@ import (
 	"sync"
 
 	"github.com/hajimehoshi/ebiten/v2/internal/atlas"
-	"github.com/hajimehoshi/ebiten/v2/internal/buffered"
 	"github.com/hajimehoshi/ebiten/v2/internal/clock"
 	"github.com/hajimehoshi/ebiten/v2/internal/debug"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
@@ -102,14 +101,9 @@ func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, update
 
 	debug.Logf("----\n")
 
-	if err := buffered.BeginFrame(graphicsDriver); err != nil {
+	if err := atlas.BeginFrame(graphicsDriver); err != nil {
 		return err
 	}
-	defer func() {
-		if err1 := buffered.EndFrame(graphicsDriver, swapBuffersForGL); err == nil && err1 != nil {
-			err = err1
-		}
-	}()
 
 	// Flush deferred functions, like reading pixels from GPU.
 	if err := c.flushDeferredFuncs(ui); err != nil {
@@ -164,6 +158,14 @@ func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, update
 		return err
 	}
 
+	if err := atlas.EndFrame(); err != nil {
+		return err
+	}
+
+	if err := atlas.SwapBuffers(graphicsDriver, swapBuffersForGL); err != nil {
+		return err
+	}
+
 	return nil
 }